First of all GCSE grades do matter because any offers given are partly based on them - so, for popular courses, as well as having A Level requirements, Unis will have a certain level of GCSEs below which, they simply won't give offers to.
Therefore, in order to get offers you need both good GCSE grades and also good predicted grades.
Once you have got these offers, if you fall short, you may well still get the place, as a number of people on this thread have said. However, if you didn't have the offer in the first place, you are unlikely to get a place on popular course if it doesn't make it to Clearing.
So, from a Uni point of view, if they haven't filled all their places on results day, with candidates meeting their offers, they have 2 choices....accept those they already made offers to but with the lower grades they actually got (and know they are likely to say YES if its their Firm choice) or turn those people down and go to Clearing in the hope of people with the higher grades....with no certainty they will appear. So given it is about funding and bums on seats, many Unis will take a candidate who hasn't quite achieved their offer grades, rather than take a risk.
What does this all mean....it means your GCSEs and predicted grades in some ways are more important than your A Levels themselves...because if you can get an offer in the first place, even if you fall slightly short, you may well be accepted anyway.
One final thing to remember is that many of the people receiving offers and actually taking up places on popular courses will have far in excess of the minimum offer requirement listed. For popular courses you really want to have stellar GCSEs (All As and A*s) and predictions in excess of the offer listed in the prospectus. In the end, if far more apply than places exist, offers will go to those with the best predictions and GCSEs.
It's good to look at unis early as a motivator for GCSE work and its definitledy good to be aware of how important the L6th year is for determining predictions...only starting to think about it in june/July of the L6th is a bit late to realise it would have been a good idea to work harder to get better predicted grades...but then of course the better schools who are used to channelling the majority onto RG courses make sure the kids know this stuff early enough to do things about it. It amazes me though how many schools don't seem to spell it all out to people soon enough - perhaps it's just because they don't expect many to get to RG unis and expect only their few elite to get in. In the end, good grades are needed, there is no getting away from it, but more people with perhaps slightly less good grades get in from schools who push them towards it.